Adsorption phenomenon involves

(I) residual attraction forces

(II) a spontaneous phenomenon

(III) a gaseous adsorbent

Adsorption is a surface phenomenon. It is the phenomenon of accumulation of large number of molecular species at the surface of liquid or solid phase in comparison to the bulk. The process of adsorption arises due to presence of unbalanced or residual forces at the surface of liquid or solid phase. These forces of attraction acting outside and are not balanced. Adsorption is a spontaneous process as Delta H becomes less negative as adsorption proceeds.

Hence the correct answer is (C).